The present invention is directed to a method for detecting the presence or absence of a bacterium resistant to a cyclic cationic polypeptide antibiotic, comprising: (a) subjecting a test sample to mass spectrometry analysis and generating a mass spectrum output; wherein said test sample comprises a bacterial membrane or a fragment thereof, wherein the fragment comprises a non- Lipid A component; (b) identifying in said mass spectrum output a first defined peak indicative of the presence of Lipid A modified by phosphoethanolamine, wherein said first defined peak is a peak present in a mass spectrum output for Lipid A modified by phosphoethanolamine and wherein said first defined peak is absent from a corresponding mass spectrum output for native Lipid A; and (c) wherein the presence of said first defined peak indicates the presence of a bacterium resistant to a cyclic cationic polypeptide antibiotic, and wherein the absence of said first defined peak indicates the absence of a bacterium resistant to a cyclic cationic polypeptide antibiotic. This method is also used in a screening method to identify an inhibitor of cyclic cationic polypeptide antibiotic resistance in a bacterium. The matrix solution can contain 2,5-dihydroxybenzoic acid and aids in the selective extraction, co-crystallisation and ionisation of native Lipid A and/or modified Lipid A as an integral part of a bacterial membrane.